1. Field of Invention
The present invention relates generally to a method for automatic removal of vertical streaks and, more specifically, to a method for automatic removal of vertical streaks by modifying image data associated with non-homogenous image elements.
2. Description of the Related Art
Scanners typically include an array of optical sensor elements and a scan area (e.g., plate of glass) where an object to be imaged by the sensor elements is positioned. An optical path including, for example, lens and mirrors, spans between the sensor elements and the scan area.
Referring to FIG. 2, a subsystem 200 of a typical scanner includes an optical sensor device 202, a lens 204, a transparent plate 206 and a calibration strip 208. The transparent plate 206 includes a scan area surface 210 over which an object 212 to be scanned is positioned. An exemplary calibration strip 208 spans across the entire scan area surface 210 and is formed from plastic with a uniform exterior color such as white.
The optical sensor device 202 is typically a linear array of optical sensor elements or photosites which convert optical images to electrical output signals. An exemplary optical sensor device 202 comprises a 2,700-bit×3 CCD (Charge Coupled Device) color linear image sensor such as the NEC μPD3720 integrated circuit which has a color filter that provides primary colors (red, green and blue) via rows of photosites 214, 216 and 218, respectively, which are arranged on the sensor device 202 as shown.
A problem with the subsystem 200 is that different photosites, due to manufacturing imperfections, do not necessarily generate the same output signal when imaging identical objects. Another problem with the subsystem 200 is that the optical path 220 (shown unfolded) between the optical sensor device 202 and the object 212 introduces inconsistencies in the output signals because the photosites at the end portions 222 and 224 of the optical sensor device 202 receive lower levels of light from an object 212 of uniform color than the photosites near the center portion 226 of the optical sensor device 202. Therefore, in order to achieve uniformity in the levels of the output signals across the optical sensor device 202, some form of compensation or calibration of the output signals is necessary. To this end, the subsystem 200 includes the calibration strip 208 which is used to calibrate the output signals of the optical sensor device 202.
Referring to FIG. 3, a functional block diagram 300 shows that output signals 302 generated by the optical sensors 202 are provided with pixel-by-pixel gain 304 to generate calibrated output signals 304. During the calibration process, the photosites of the optical sensor device 202 image the uniformly colored calibration strip 208 before the object 212 to be scanned is positioned on the scan area surface 210. Each photosite in the scanner is “queried” to determine how much light it “sees”. Across the optical sensor device 202, from the left end 222 to the right end 224, the output signals 302 appear, for example, as shown in FIG. 4. In order to achieve uniformity in the levels of the output signals across the optical sensor device 202, a “proportionate” pixel-by-pixel gain 304 as shown in FIG. 5 is applied to the output signals 302. The term “proportionate” means an inversion or other appropriate function of the output signals 302 such that the calibrated output signals 304 appear as the uniform output level shown in FIG. 6. By way of example, suppose an average photosite reports a value of 100. If one photosite reports a lower value—say 50—then the amplification for that one photosite will be set twice as high as the amplification for the average photosite. After the calibration process is completed, the pixel-by-pixel gain 304 is saved, for example, in firmware of the scanner, and applied during subsequent scanning. Thus, the net signal from the photosite and its amplification are the same for all photosite-amplification pairs.
Even though each photosite gets a “customized” amplification, unfortunately, this does not accommodate a situation where an optical obstruction is positioned between the calibration strip 208 and the scan area surface 210 during the calibration process. The term “optical obstruction” means an object which has any effect on light transmitted therethrough. Optical obstructions include, but are not limited to, paper dust, plastic dust, skin particles, metal particles and glass particles.
Referring again to FIG. 2, the subsystem 200 is shown with optical obstructions “A”, “B”, “C” and “D” positioned between the optical sensor device 202 and the calibration strip 208. More specifically, the optical obstructions “A”, “B”, “C” and “D” are positioned, respectively, on the scan area surface 210, in the optical path 220, in the optical path 220 sufficiently near the scan area surface 210 to be illuminated by a light source (not shown), and on the optical sensor device 202. The optical obstructions “A”, “B” and “D” are dark debris which are light-absorbing, i.e., tending to absorb light. The optical obstruction “C” is reflective. During the calibration process, when these optical obstructions are present, the output signals 302, from the left end 222 to the right end 224 of the optical sensor device 202, appear, for example, as shown in FIG. 7. In order to achieve uniformity in the levels of the output signals across the optical sensor device 202, a “proportionate” pixel-by-pixel gain 304 as shown in FIG. 8 is applied to the output signals 302. As shown in FIG. 9, a uniform photosite output signal level with proportionate gain applied is the result of the calibration process. However, if the optical obstruction “A” is displaced from the optical path 220, for example, by an object 212 moving across the scan area surface 210, the calibrated output signal levels will then appear as shown in FIG. 10 with a large spike corresponding to the photosite that was imaging the optical obstruction “A” during the calibration process. As a result, during scanning, this erroneously high gain causes all scan data from that photosite to have a higher signal than it should. The net effect is that there is a bright vertical line in the scan, copy or fax output which runs the entire length of the image.
Vertical lines or streaks in scan, copy or fax output can also be caused by transient debris or optical obstructions which, for example, fall into the optical path 220 of the optical sensor device 202 after the calibration has occurred. Vertical lines or streaks can also be caused by dead pixels, hot pixels, photosite offset non-uniformities and other photosite and imaging system malfunctions and non-uniformities. Thus, a need exists for a method for eliminating vertical lines or streaks in scan data.